The present invention relates to a seat belt retractor with load limiting.
A seat belt retractor generally comprises a cylindrical bobbin or spool with a circular cross-section. Seat belt webbing is attached to and wound around the spool. The spool is mounted on a spool shaft to be rotatable in the retractor to wind in webbing under action of a retractor spring and to pay out webbing under the influence of relatively gentle forwardly directed movement of a vehicle occupant, for example normal movement associated with reaching the controls for a radio or a window or to reach a glove compartment or door pocket. In a crash the more extreme momentum of the vehicle occupant activates a crash sensor that locks the spool against rotation and prevents forward motion of the vehicle occupant and collisions with the interior fixtures of the vehicle. However, this sudden locking of the seat belt spool in a crash can sometimes cause injury to the vehicle occupant due to sudden impact of the torso with the belt webbing. In recent years this problem has been recognized and some solutions proposed.
One known solution is to effect load limiting by interposing a plastically or elastically deformable member in the force path. For example, a torsion bar may be incorporated into the spool. A torsion bar is made of steel that twists when high torque is applied to it and can rotate up to seven or eight times before breaking. It is connected to both ends of a split spool. One end of the spool is held against rotation by a locking ring engaged by a load bearing pawl in a crash. The other end of the spool is prevented from rotating by its connection to the locking ring via the torsion bar, but when the crash forces exceed a predetermined level the torsion bar will twist and allow a limited controlled further pay-out of belt webbing. The additional pay-out of belt webbing is generally in proportion to the momentum of the vehicle occupant at the moment of a crash. This pay-out of belt webbing lessens the otherwise severe restraining forces on the vehicle occupant at high vehicle speeds, especially during the initial moments of a crash.
However torsion bars are complex and expensive to make, and are not easily adaptable to different conditions, such as different vehicle characteristics, or different sizes and weights of vehicle occupants. There is a need for a cheaper and less complex load limiting system.
There is provided in accordance with one aspect of the invention a seat belt retractor comprising: a spool mounted for rotation in the retractor; a locking ring attached to one end of the spool; a locking pawl for locking the locking ring against rotation in a crash; and load limiting means for allowing further pay-out of the seat belt webbing under the influence of a vehicle occupant""s forward momentum after the locking ring has locked, wherein the load limiting means comprises a length of wire and a winding drum attached to one of the spool and the locking ring and a pin fixed to the other of the spool and the locking ring, wherein the wire is coiled around the pin and arranged so that when there is relative movement between the spool and the locking ring the wire is pulled around the pin and is wound onto the winding drum.